Genetic evidence for an interferon-antagonistic function of Rift Valley fever virus nonstructural protein NSs

Rift Valley fever vints (RVFV), a phlebovirus of the family Bunyaviridae, i s a major public health threat in Egypt and sub-Saharan Africa. The viral a nd host cellular factors that contribute to RVFV virulence and pathogenicit y are still poorly understood. All pathogenic RVFV strains direct the synth esis of a nonstructural phosphoprotein (NSs) that is encoded by the smalles t (S) segment of the tripartite genome and has an undefined accessory funct ion. In this report, we show that MP12 and clone 13, two attenuated RVFV st rains with mutations in the NSs gene, were highly virulent in IFNAR(-/-) mi ce lacking the alpha/beta interferon (IFN-alpha/beta) receptor but remained attenuated in IFN-gamma receptor-deficient mice. Both attenuated strains p roved to be excellent inducers of early IFN-alpha/beta production. In contr ast, the virulent strain ZH548 failed to induce detectable amounts of IFN-a lpha/beta and replicated extensively in both IFN-competent and IFN-deficien t mice. Clone 13 has a defective NSs gene with a large in-frame deletion. T his defect in the NSs gene results in expression of a truncated protein whi ch is rapidly degraded. To investigate whether the presence of the wild-typ e NSs gene correlated with inhibition of IFN-alpha/beta production, we infe cted susceptible IFNAR(-/-) mice with S gene reassortant viruses. When the S segment of ZH548 was replaced by that of clone 13, the resulting reassort ants became strong IFN inducers, When the defective S segment of clone 13 w as exchanged with the wild-type S segment of ZH548, the reassortant virus l ost the capacity to stimulate IFN-alpha/beta production. These results demo nstrate that the ability of RVFV to inhibit IFN-alpha/beta production corre lates with viral virulence and suggest that the accessory protein NSs is an IFN antagonist.